1. Field of the Invention
The present invention relates to a lifting and steering device for a steering wheel of a vehicle, of the type comprising a main fixed tubular body which extends substantially vertically and which has an upper end closed, a second tube which is disposed coaxially inside the main tubular body and is slidably mounted therein and projects beyond its lower end, a wheel support fixed rigidly to the lower end of the second tube, a shaft which is mounted for rotation in the closed upper end of the main tubular body and which extends axially inside this latter and the second tube, first coupling means for coupling the upper end of the shaft to a steering control of the vehicle, a second coupling means for coupling the second tube to the shaft so as to interlock them for rotation while allowing an axial movement of the second tube with respect to the shaft and a hydraulic cylinder for axially moving the second tube with respect to the shaft.
2. Description of the Prior Art
Lifting and steering devices of the above mentioned kind are used for example in some vehicles which have a high center of gravity and which are equipped with a hydraulic tilt correcting system which acts selectively on each of the wheels of the vehicle to correct its level position when it is travelling over sloping ground. This is particularly the case in a certain number of harvesting machines such as grape harvesting machines, coffee harvesting machines, machines for harvesting blackcurrants, raspberries and other berries.
A lifting and steering device is already known of the above described type (see U.S. Pat. No. 4,340,237), in which the main tubular body and the second tube form respectively the cylinder and the piston rod of the hydraulic cylinder. The piston of the cylinder is mounted for sliding inside the main tubular body and it is rigidly fixed to the upper end of the second tube whose lower end is sealingly closed in relation to the fluid. Furthermore, the part of the shaft which extends inside the main tubular body and the second tube has a non circular section and passes through a central aperture in the piston, which has a form corresponding to the non circular section of the shaft and which forms with this latter the second above mentioned coupling means.
In operation, the high forces applied to the wheel (driving, steering) by reaction with the ground introduce high stresses so deformations in the sliding parts of the lifting and steering device. In the lifting and steering device of the U.S. Pat. No. 4,340,237, these stresses and deformations cause rapid wear of the seals between the main tubular body and the piston on the one hand, and between the main tubular body and the second tube forming the piston rod, on the other. The result is leaks of pressurized fluid which adversely affect the operation of the lifting and steering device. In addition, the lower part of the second tube which slides in the lower end of the main tubular body and which extends outwardly thereof, is located at the level of the wheel and is then vulnerable to abrasion by the ground, the sand or the mud which, during operation, is thrown up by the wheel. The earth, sand and mud may thus cause wear of the seals between the main tubular body and the second tube and/or scratches on the outer surface of the second tube, which adversely affect the sealing of the hydraulic cylinder.